White Etching Cracks Research Articles

Formation of nanosized grain structure in martensitic 100Cr6 bearing steels upon rolling contact loading studied by atom probe tomography

To understand the origin of white etching cracks (WECs), a systematic microstructural characterisation in the regions affected from the near-surface region down to the subsurface layers where WECs occur is necessary. As a starting point, we focus on the near-surface region of an axial thrust bearing, made of martensitic 100Cr6 steel, to study the influence of rolling contact loading on the microstructure and the resulting distributions of the major alloying elements C and Cr using atom probe tomography. We find that upon rolling contact loading the original plate-like martensitic structure evolves into a nanosized equiaxed grain structure with C segregation up to 5 at.-% at the grain boundaries. Cementite particles, located at grain boundaries and triple junctions, undergo spheroidisation. The originally homogeneously distributed Cr becomes enriched in spheroidised cementite particles. The microstructural changes give strong hints that rolling contact loading induces plastic deformation and an increased temperature on the near-surface region.This paper is part of a Themed Issue on Recent developments in bearing steels.

Investigating the Process of White Etching Crack Initiation in Bearing Steel

White etching cracks (WECs) have been identified as a dominant mode of premature failure within wind turbine gearbox bearings. Though WECs have been reported in the field for over a decade, the conditions leading to WECs and the process by which this failure culminates are both highly debated. In previously published work, the generation of WECs on a benchtop scale was linked to sliding at the surface of the test sample, and it was also postulated that the generation of WECs was dependent on the cumulative energy that had been applied to the sample over the entirety of the test. In this paper, a three-ring-on-roller benchtop test rig is used to systematically alter the cumulative energy that a sample experiences through changes in normal load, sliding, and run-time, in an attempt to correlate cumulative energy with the formation of WECs. It was determined that, in the current test setup, the presence of WECs can be predicted by this energy criterion. The authors then used this information to study the process by which WECs initiate. It was found that, under the current testing conditions, the formation of a dark etching microstructure precedes the formation of a crack, and a crack precedes the formation of white etching microstructure.

Formation of white etching cracks at manganese sulfide (MnS) inclusions in bearing steel due to hammering impact loading

Wind turbine gearbox bearings (WTGBs) are failing prematurely, leading to increased operational costs of wind energy. Bearing failure by white structure flaking (WSF) and axial cracking may both be caused by the propagation of white etching cracks (WECs) and have been observed to cause premature failures; however, their damage mechanism is currently not well understood. Crack initiation has been found to occur at subsurface material defects in bearing steel, which may develop into WECs. One hypothesis for WEC formation at these defects, such as non-metallic inclusions, is that repetitive impact loading of a rolling element on a bearing raceway, due to torque reversals and transient loading during operation, leads to high numbers of stress-concentrating load cycles at defects that exceed the material yield strength. In this study, a number of tests were carried out using a reciprocating hammer-type impact rig. Tests were designed to induce subsurface yielding at stress concentrating manganese sulfide (MnS) inclusions. The effects of increasing surface contact stress and number of impact cycles, with and without surface traction, were investigated. Damage adjacent to MnS inclusions, similar to that observed in a failed WTGB raceway, was recreated on bearing steel test specimens. It has been found that increasing the subsurface equivalent stresses and the number of impact cycles both led to increased damage levels. Damage was observed at subsurface equivalent stresses of above 2.48 GPa after at least 50,000 impact cycles. WECs were recreated during tests that applied surface traction for 1,000,000 impacts.

The Influence of Sliding and Contact Severity on the Generation of White Etching Cracks

White etching cracks (WECs) have been identified as the dominant mechanism of premature failure for bearings within wind turbine gearboxes. Though WECs have been observed in the field for over a decade, the exact mechanisms which lead to this failure are still debated, and benchtop replication has proven difficult. In previously published work, WECs have been replicated only through the use of component level test rigs, where complete bearings are tested. In these tests, the factors that are thought to drive the formation of WECs, such as slide-to-roll ratio (SRR) and lubricant film thickness, cannot not be easily altered or controlled. In this paper, WECs have been replicated on a three ring on roller, benchtop test rig, which allowed for a direct investigation into the influence that SRR magnitude, sliding direction, and the lubricant film thickness have on surface failures and WEC generation. It was determined that WECs were formed in samples that experienced −30 % SRR at various lubrication conditions; however, at lower levels of negative SRR and positive SRR up to 30 %, no white etching cracks were observed.

The Numerical Model of Electrothermal Deformations of Carbides in Bearing Steel as the Possible Cause of White Etching Cracks Initiation

Despite the ongoing debates on influence of hydrogen uptake and penetration in the steel, pulsed and extraordinary fatigue on white etching cracks (WEC) formation in bearing steel SAE52100, the present paper proposes an alternative hypothesis on electrothermal initiation of the WEC. The hypothesis points to differences between electrical and thermal properties of elements of steel microstructure that lead sequentially to redistribution of current, resistivity heating, thermal expansion and deformations of the carbide particle. Appearance of a nano-void is also predicted by the model in the cases of the martensite and the bainite structures. The model also predicts higher probability of the WEC formation for the bainitic steel.

Characterisation of white etching crack damage in wind turbine gearbox bearings

White etching cracks (WECs) have been identified as a main failure mode of wind turbine gearbox bearings (WTGBs). This study reports an investigation of the destructive sectioning of a failed low speed planetary stage WTGB and the damage found at manganese sulphide (MnS) inclusions. The bearing inner raceway was sectioned through its circumferential and axial directions in order to compare the damage around inclusions in different directions. 112 damage initiating inclusions were catalogued and their properties were investigated.WECs were found attached to MnS inclusions of lengths 3–45µm at depths of up to 630µm from the bearing raceway surface and at a wide range of angles of orientation. Damage at MnS inclusions included internal cracking of the inclusions, separation from the surrounding steel matrix, crack initiation and WEC initiation. Evidence has been found to support the theory that WECs are subsurface-initiated by MnS inclusions, but that butterfly cracks with wings propagating at 30–50° from parallel to the raceway surface are not necessarily the same features as MnS inclusion-initiated WECs. Shorter inclusions were found to initiate longer WECs, as were the inclusions that were closer to parallel to the raceway surface in axially sectioned samples.

Observations of the effect of varying Hoop stress on fatigue failure and the formation of white etching areas in hydrogen infused 100Cr6 steel rings

White etching cracks (WECs) in wind turbine gearbox bearings have been studied previously. Rolling contact fatigue (RCF) tests are conducted on 100Cr6 bearing steel rings, in this study, to generate WECs like those found in wind turbine bearings. This research studies the effect of two different levels of tensile Hoop stresses on the failure life and formation of WECs in the rings. The rings are pre-charged with hydrogen before RCF tests are conducted. It is found that these rings experience incremental fatigue failure, followed by a sudden rapid failure. The fractography, Reflecting Light Microscopy (RLM) and Scanning Electron Microscopy (SEM) results are presented in this paper.

Understanding white etching cracks in rolling element bearings: Formation mechanisms and influent tribochemical drivers

Among prevalent tribological failures in rolling element bearings, a peculiar rolling contact fatigue mode has been defined as white etching cracks, which correspond to broad subsurface three-dimensional branching crack networks bordered by white etching microstructure. White etching cracks tend to appear before other conventional rolling contact fatigue microstructural alterations and eventually lead to premature flaking or radial cracking that remains unpredictable using fatigue life estimations. Even though they have been reported for several decades, occurrences present no common evident denominator and are delicate to reproduce on laboratory test rigs without artificial hydrogen charging. In this study, white etching crack reproductions on two different standard endurance test rigs are compared and analyzed in order to, firstly, propose, an update of the understanding of white etching crack surface affected tribochemical formation mechanisms, and secondly, to identify influent tribochemical and mechanical drivers. If white etching cracks are associated to different combinations of specific non-self-sufficient macroscopic drivers depending on the application or the test rig, evidences demonstrate that they often come down to similar phenomena at a microscopic tribological scale that should all be mastered for efficient white etching crack reproduction and countermeasure design.

White Etching Crack Root Cause Investigations

White etching crack (WEC) failure is distinct to classical fatigue and driven by the composition of lubricants under special loading conditions; for example, slippage and electricity. The white etching area (WEA) within WEC contains carbon supersaturated ferrite (bcc-iron) and carbides, with a size of a few nanometers. This article presents investigations supporting the hypothesis that WEC processes start within a failure-free period by successive accumulation of a structural distortion. This can be measured by acoustic emission. Failure statistics show a steep ascent in the Weibull diagram (ß values beyond 1) leading to the assumption that WEC processes start unsuspicious, as one would see as a failure-free period, but imply a hidden subsurface accumulation of material defects. It is suggested and supported by the evidence presented within this article that WEC is neither related to the presence of nonmetallic inclusions nor related to other impurities in the steel. Instead, the failure is a sequence and accumulation of plastic deformations in the microstructure. Within the SAE 52100 material as discussed in this article, this accumulation is located in the microstructure around cementite, seen in a turn of hard magnetization toward soft magnetization proven by Barkhausen noise measurements. This decay is caused by the plastic deformation of such domains. Distortions in the vicinity of a cementite first would lead to carbon supersaturation by diffusion processes and later to a plastic deformation of the carbides. In the end, the complete distorted region will release the accumulated energy by downsizing the microstructure toward WEC.

White-Etching Matter in Bearing Steel. Part I: Controlled Cracking of 52100 Steel

Although most of the research performed in bearing steel metallurgy aims to prevent crack nucleation and propagation, some applications require the exact opposite in order to study the role that disconnected surfaces inside the bulk material play when load is applied, or when fluids entrapped in surface cracks propagate tensile stresses or exacerbate corrosion. Four heat treatments have been designed to create controlled arrays of crack types and distributions in quenched and untempered steel normally used in the manufacture of bearings. The varieties of cracks studied include sparsely distributed martensite-plate cracks, fine-grain-boundary cracks, abundant martensite-plate cracks, and surface cracks. The intention was to create samples which can then be subjected to appropriate mechanical testing so that phenomena such as the appearance of “white-etching areas” or “white-etching cracks,” crack-lubricant interactions, or hydrogen trapping can be studied further.

Confirming subsurface initiation at non-metallic inclusions as one mechanism for white etching crack (WEC) formation

White etching crack (WEC) formation beneath the contact surface in steel rolling element bearings causes a premature wear failure mode called white structure flaking (WSF). The drivers and initiation mechanisms are contested. Extensive work previously conducted by the authors showed strong evidence for one mechanism of WEC initiation being subsurface at inclusions. This paper conducts further work for final verification. Rolling contact fatigue testing of bearings on a FAG-FE8 test rig was conducted. Serial sectioning was used to map WECs in their entirety that formed under non-hydrogen charged conditions for the first time. Evidence from this paper and previous works by the authors confirms that subsurface initiation of WECs at non-metallic inclusions is at least one mechanism of WEC formation.

Formation mechanisms of white etching cracks and white etching area under rolling contact fatigue

The formation of white etching cracks in the 1 mm zone beneath the contact surface in steel rolling element bearings causes a premature wear failure mode called white structure flaking. The formation drivers of white etching cracks are contested, as are the initiation and propagation mechanisms of the cracks. Hydrogen diffusion into bearing steel sourced from the hydrocarbon lubricant or water contamination and transient operating conditions have been suggested as formation drivers. Extensive work has been conducted at Southampton to further understanding of white structure flaking and this paper summarises these evidences and the conclusions made. Serial sectioning has been used to map subsurface wear volumes of wind turbine gearbox bearings from service and large-scale test rigs, test specimens/bearings from laboratory under hydrogen charged conditions and non-hydrogen charged conditions. The process involves polishing of cross sections of test specimens/bearings at ∼3–5 µm material removal intervals typically over hundreds of slices, and this was used to map white etching cracks in their entirety for the first time. Serial sectioning has allowed a comprehensive investigation of the initiation and propagation mechanisms of white etching cracks and thresholds for their formation with respects to concentration of diffusible hydrogen, contact pressure and number of rolling cycles. From these studies it has been found that white etching cracks can form by subsurface crack initiation at inclusions under hydrogen charged and non-hydrogen charged conditions; hence it has been confirmed that this is one mechanism of WEC formation. Small/short sized sulfide inclusions, globular manganese sulfide + oxide inclusions and small globular oxide inclusions between ∼1 µm and 20 µm in diameter/length predominated as crack initiators. In addition, detailed focused ion beam/transmission electron microscopic studies have been conducted to enhance the understanding of butterfly crack and white etching area formation mechanisms.

Understanding white etching cracks in rolling element bearings: The effect of hydrogen charging on the formation mechanisms

Among prevalent tribological failures, notably in rolling element bearings for wind turbines, an unusual rolling/sliding contact fatigue failure mode has been identified as white etching cracks. White etching cracks are broad subsurface three-dimensional branching crack networks bordered by white etching microstructure, eventually leading to flaking. Reproduction of the failure mode on standard rolling element bearings test rigs has not been mastered yet except with artificial hydrogen charging. Even though these failures have been reported for several decades, there is no evident common denominator in different occurrences. Hence, initiation and propagation mechanisms are not yet fully understood in application. Analyses of the contact conditions of a standard rolling element bearings test rig reproducing white etching cracks on standard and hydrogen precharged inner rings reveal that hydrogen charging seems to modify the white etching cracks initiation mechanism. Based on fractographs, serial sectioning, and scanning electron microscopic analyses, surface initiation and propagation mechanisms are proposed, including influent drivers and possible preventive techniques.

Investigations of Bearing Failures Associated with White Etching Areas (WEAs) in Wind Turbine Gearboxes

A critical problem for wind turbine gearboxes is failure of rolling element bearings where axial cracks form on the inner rings. Metallurgical analyses show that the failure mode is associated with microstructural alterations manifested by white etching areas (WEAs) and white etching cracks (WECs). This article presents field experience from operating wind turbines that compares performance of through-hardened and carburized materials. It shows that through-hardened bearings develop WEA/WECs and fail with axial cracks, whereas carburized bearings do not. In another comparison of two rotor bearings with different carburized metallurgies, one bearing developed WEA/WECs and failed by macropitting, whereas the other bearing did not develop WEAs or WECs and did not fail. The field experience shows that a carburized bearing that has a core with low carbon content, high nickel content, greater compressive residual stresses, and a higher amount of retained austenite provides higher fracture resistance and makes carburized bearings more durable than through-hardened bearings in the wind turbine environment.

Service Loading Analysis of Wind Turbine Gearbox Rolling Bearings Based on X-Ray Diffraction Residual Stress Measurements

Rolling bearings in wind turbine gearboxes occasionally fail prematurely by so-called white etching cracks. The appearance of the damage indicates brittle spontaneous tensile stress induced surface cracking followed by corrosion fatigue driven crack growth. An X-ray diffraction based residual stress analysis reveals vibrations in service as the root cause. The occurrence of high local friction coefficients in the rolling contact is described by a tribological model. Depth profiles of the equivalent shear and normal stresses are compared with residual stress patterns and a relevant fracture strength, respectively. White etching crack failures are reproduced on a rolling contact fatigue test rig under increased mixed friction. Causative vibration loading is evident from residual stress measurements. Cold working compressive residual stresses are an effective countermeasure.

Application of Complementary Techniques for Advanced Characterization of White Etching Cracks

Abstract White etching crack (WEC) networks were characterized in heavy loaded bearings for wind turbines. Both conventional techniques as reflected light microscopy and (scanning and transmission) electron microscopy as well as electron backscatter diffraction and ion channelling contrast imaging were applied. The complementary use of the techniques in unravelling the complicated failure mechanisms is explored in the present work.

White etching crack (WEC) investigation by serial sectioning, focused ion beam and 3-D crack modelling

White structure flaking (WSF) as a premature wear failure mode in steel rolling element bearings is caused by white etching cracks (WECs) that form in the 1mm zone beneath the contact surface. Hydrogen diffusion into raceways and transient operating conditions have been suggested as drivers of WSF. The initiation and propagation mechanisms for WEC formation are not well understood. This study elucidates WEC initiation processes in hydrogen charged 100Cr6 bearing steel by carrying out two-roller RCF testing. The application of serial sectioning enabled mapping and modelling of entire independent WECs and revealed critical information about initiators. The application of FIB tomography verified initiation mechanisms. The results show strong evidence for subsurface initiation of WECs from inclusions.

Effect of hydrogen on butterfly and white etching crack (WEC) formation under rolling contact fatigue (RCF)

White structure flaking (WSF) as a premature wear failure mode in steel rolling element bearings is caused by white etching cracks (WECs) formed in the 1mm zone beneath the contact surface. Hydrogen release and diffusion into the bearing steel during operation and transient operating conditions have been suggested as drivers of WSF. The presence of diffusible hydrogen in steel under rolling contact fatigue (RCF) has been shown to strongly promote the formation of WEA/WECs. However, the initiation and propagation mechanisms, as well as the thresholds for WEC formation, are not well understood. This study uses hydrogen charging of 100Cr6 bearing steel rollers prior to testing on a two-roller RCF rig to enable WEA/WEC formation under service realistic loading. This study focuses on the influence of the concentration of diffusible hydrogen, the magnitude of the contact load and the number of rolling cycles on the formation of white etching features (butterflies, WEA/WECs) which are determined by a serial sectioning process. The formation of butterflies was found to be independent of concentration of diffusible hydrogen with the test parameters used, but dependent on contact pressure and number of rolling cycles up to a threshold. WEA/WEC formation thresholds were found at certain values of the concentration of diffusible hydrogen, contact pressure and number of rolling cycles. The results also show evidence for a subsurface initiation mechanism of WECs from non-metallic inclusions. It is proposed that one mechanism of WEC formation is due to multiple linking of extended butterflies or small WECs in the subsurface to form larger WEC networks that eventually propagate to the surface resulting in WSF.

Serial sectioning investigation of butterfly and white etching crack (WEC) formation in wind turbine gearbox bearings

Premature wind turbine gearbox bearing failures in the form of white structure flaking (WSF) can occur in as little as 6–24 months of operation. WSF is not fully understood but is thought to be due to hydrogen release and diffusion into the bearing steel and/or transient operating conditions not fully understood. The initiation mechanisms of white etching cracks (WECs) are contested, where amongst others mechanisms, subsurface initiation at non-metallic inclusions (perhaps associated with extension of butterfly cracks) and surface crack initiation are cited. For the first time this study applies serial sectioning to map WEC networks in wind turbine gearbox bearings to elucidate WEC initiation mechanisms. A comparison is made between WEC data for inner rings of an industrial transient test gearbox bearing and a planet bearing that spalled in service. It is proposed that one mechanism of WEC formation in wind turbine gearbox bearings is due to subsurface WEC initiation from inclusions, either in a butterfly manner or non-butterfly manner; where these small WECs link together to form larger WEC networks, these eventually propagating to the surface resulting in WSF. Small size/length inclusions were found to be likely WEC initiators, therefore the data suggests that steel cleanliness standards analysing inclusion density (as opposed to maximum inclusion lengths) are more relevant in understanding butterfly/WEC initiation in wind turbine gearbox bearings. However standards used should be able to differentiate pure sulfides from sulfides+oxide encapsulations and record inclusions that are only a couple of mircometer’s in length/diameter.

Letter to the Editor

Correspondence regarding the role of white etching cracks and related features in the failure processes of bearing steels, with responses from the authors